1. Field of the Invention
The invention relates to a short-arc, ultra-high pressure discharge lamp in which the mercury vapor pressure during operation is at least 150 atm. The invention also relates especially to a short-arc, ultra-high pressure discharge lamp which is used as the back light of a liquid crystal display device and a projector device using a DMD (digital mirror device), like a DLP (digital light processor) or the like.
2. Description of the Prior Art
In a projector device of the projection type, there is a demand for illumination of the images uniformly onto a rectangular screen with sufficient color reproduction. Thus, a metal halide lamp which is filled with mercury and a metal halide is used as the light source. Furthermore, recently smaller and smaller metal halide lamps and more and more often spot light sources have been produced and lamps with extremely small distances between the electrodes have been used in practice.
Against this background, recently, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with 150 atm, have been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is compressed) and a major increase of the light intensity is desired. One such ultra-high pressure discharge lamp is disclosed in JP-OS HEI 2-148561 (corresponding to U.S. Pat. No. 5,109,181) and JP-OS HEI 6-52830 (corresponding to U.S. Pat. No. 5,497,049).
These ultra-high pressure discharge lamps have a light emitting part in which there are a pair of electrodes, and side tube parts on opposite ends thereof. In the respective side tube part, one end of the electrode and a metal foil are welded to one another. Due to the demand for reducing the size of lamps and for a spot light source, there is a demand for an electrode with an upholding part with a small outside diameter, for example, from 0.2 mm to 1.0 mm, and furthermore, a demand for an extremely small electrode distance, i.e., an extremely small arc length, for example, of roughly 0.5 mm to 2.0 mm.
FIGS. 6(a) & 6(b) each schematically show the arrangement of the tip area of the electrode, enlarged. Here, there are a cathode 11 and an anode 12 opposite at a small distance from one another. Between the two electrodes, an arc bright spot P is formed. However, as is shown in FIG. 6(b), there arc cases in which the cathode 11′ is shifted (the position deviates from a given position). In this case, the distance to the anode 12 is increased, i.e., the distance between the electrodes. In addition, the position of the arc bright spot P′ changes. Such a change of the electrode distance and the position of the arc bright spot reduces the efficiency of the light to a large degree because a projector device or the like is optically engineered based on given adjustment values.
FIGS. 7(a) & (b) each schematically show the electrode displacement. FIG. 7(a) shows the state in which the cathode on the metal foil is shifted during resistance welding of the cathode 11 to the metal foil 13, due to contact of the welding rod with the cathode or the metal foil. Since the diameter of the cathode is 0.2 mm to 1.0 mm, therefore, is extremely small, as was described above, the cathode often moves during contact with the welding rod. FIG. 7(b) shows the state in which the cathode tip is also shifted as a result of bending of the metal foil itself. This occurs in a process in which the electrode assembly, after resistance welding of the metal foil to the cathode, is hermetically scaled in silica glass. The metal foil bends due to the flow property of the silica glass in the molten state. This description above relates above to the cathode, but it likewise applies to the anode.
The above described disadvantage which occurs due to shifting of the electrode tip is disadvantageous in a discharge lamp with an extremely small electrode distance of 0.5 mm to 2.0 mm. In a discharge lamp with a large electrode distance, for example of at least 5 mm, it never develops into a major disadvantage, e.g., the arc bright spot being shifted or the like, even if the electrode tip is more or less shifted.
In an ultra-high pressure discharge lamp as the light source of a projector device of the projection type, the amount of light decreases to a large degree even with a small electrode displacement. In this respect, there is a completely new task which never occurs in a conventional discharge lamp.
Since in such an ultra-high pressure discharge lamp the pressure within the arc tube becomes extremely high during operation, it is necessary to place the silica glass comprising these side tubes parts, the electrodes and metal foils in a sufficient amount, and moreover, directly and tightly adjoining one another in a secure manner in the side tube parts which extend from opposite sides of the arc tube. If they are not relatively tightly adjoining one another, the added gas escapes or cracks form.